X-rays are widely used in materials analysis systems. For example, X-ray spectrometry is an economical technique for quantitatively analyzing the elemental composition of samples. The irradiation of a sample by high energy electrons, protons, or photons ionizes some of the atoms in the sample. These atoms emit characteristic X rays, whose wavelengths depend upon the atomic number of the atoms forming the sample, because X-ray photons typically come from tightly bound inner-shell electrons in the atoms. The intensity of the emitted X-ray spectra is related to the concentration of atoms within the sample.
Typically, the X-rays used for materials analysis are produced in an X-ray tube by accelerating electrons to a high velocity with an electro static field, and then suddenly stopping them by a collision with a solid target interposed in their path. The X-rays radiate in all directions from a spot on the target where the collisions take place. The X-rays are emitted due to the mutual interaction of the accelerated electrons with the electrons and the positively charged nuclei which constitute the atoms of the target. High-vacuum X-ray tubes typically include a thermionic cathode, and a solid target. Conventionally, the thermionic cathode is resistively heated, for example by heating a filament resistively with a current. Upon reaching a thermionic temperature, the cathode thermionically emits electrons into the vacuum. An accelerating electric field is established, which acts to accelerate electrons generated from the cathode toward the target. A high voltage source, such as a high voltage power supply, may be used to establish the accelerating electric field. In some cases the accelerating electric field may be established between the cathode and an intermediate gate electrode, such as an anode. In this configuration, a substantially field-free drift region is provided between the anode and the target. In some cases, the anode may also function as a target.
Unfortunately, resistively heated cathodes suffer several disadvantages. Thermal vaporization of the tube's coiled cathode filament is frequently responsible for tube failure. Also, the electric current used for heating is substantial and readily affects the electric field in front of the cathode where the electron stream is formed. This creates undesirable electron stream patterns which decrease the efficiency of the source. Generating and delivering the filament current to the source further creates challenges and potential interference with the high voltage source in miniaturized applications.
In the field of medicine and radiotherapy, an optically driven (i.e. Laser) therapeutic radiation source has been previously disclosed. This optically driven therapeutic radiation source uses a reduced-power, increased efficiency electron source, which generates electrons with minimal heat loss. With the optically driven thermionic emitter, electrons can be produced in a quantity sufficient to provide the electron current necessary for generating therapeutic radiation at the target, while significantly reducing power requirements.
For materials analysis systems, where output requirements are higher, there is a need for high-efficiency, miniaturized X-ray sources.